This invention relates generally to rocker arms for operating valves in internal combustion engines and other machines and more particularly to individually mounted stamped steel rocker arms with anti-friction pivot bearings.
In the past, rocker arms for transmitting camshaft motion to valves in machinery consisted of arms pivoted on a shaft which was mounted on studs projecting from the cylinder head in which the valves were installed. The quest for increased efficiency and weight reduction, especially in internal combustion engines, led to introduction of individually mounted rocker arms.
Most individually mounted rocker arms consist of a rocker arm with an axial pivot body which is mounted on the engine with a bolt or other fastener. The pivot body supports bearings on a pivot axis on which the rocker arm is pivoted. The rocker arm has bearing support areas which are thickened wall areas surrounding the pivot axis of the rocker arm. The rocker arms are usually made from investment castings which are very strong and generally of high quality, although they are quite heavy and costly.
Efforts to reduce engine weight and cost have led engine manufacturers to attempt to make rocker arms from thin steel stampings. Such rocker arms provide lower weight and cost, but they introduce other difficulties not encountered with cast counterparts. Since there are some tight radii and deep draws necessary in light weight stamped rocker arms, it is necessary to use the thinnest material possible for such stampings. The benefits of easier forming and lower weight obtained by use of the thin material are offset by the decreased rigidity of the formed body and the severely reduced bearing support areas in the formed walls of the rocker arm. The bearing support area is critical; because if it becomes too small, the bearings may fall out or become misaligned, either of which would be catastrophic to the machine or engine. However, if the material is thickened enough to provide equivalent bearing support area to that achieved in cast rockers, forming difficulty is increased by a greater-than-linear factor, and any weight reduction advantage is diminished or completely lost. Some improvement has been achieved in bearing support housings of stamped rocker arms by extruding such housings in the walls of the rocker arm. Although more difficult and costly to make than rocker arms without extruded supports, such rocker arms provide improved bearing support over that obtained without extrusion.
Even with the bearing support problem solved, stamped rocker arms, when compared to cast rocker arms, have significantly lower strength and endurance. Because they do not have the gussets and other cast-in reinforcements of castings, stamped rocker arms have no lateral support for the free edges of their sidewalls. Thus, if they are stamped from material thin enough to provide very great weight reduction, they are subject to buckling under compressive loading of the upper lateral walls in service. They may also experience bearing walk resulting from such buckling, which causes the bearing support areas, even those with extruded bearing housings, of the rocker arm to become misaligned with the pivot axis. This presents the dilemma in which light-weighting is desired but is limited by bearing support considerations and compressive rigidity. Formability, on the other hand, dictates using the thinnest material possible in order to permit severe forming strains.
The foregoing illustrates limitations known to exist in present stamped steel rocker arms. Thus, it would clearly be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.